Conventionally, there has been proposed an electrically driven valve arranged to selectively open or close the valve by rotating the valve body of the valve through driving by a motor, and the electrically driven valve of this kind is generally so constructed that, during stopping of the motor, a rotary shaft of the motor is fixed for non-rotation by a braking mechanism provided within the motor so that the valve body of the valve may not be rotated by the pressure of a fluid passing through a flow passage.
Accordingly, in the case where it becomes impossible to control the valve for opening and closing due to incapability of energization of the motor by power failure or some other troubles, the valve body of the valve remains in the state before the interruption of the energization, and therefore, when the energization becomes impossible, with the valve body left in the opened state, there has been such a danger that a tank may be overflowed. Accordingly, it has been a conventional practice that an operator manually closes the valve simultaneously as the trouble takes place, but this practice is not fully satisfactory from the viewpoint of security, since it is almost impossible to control a plurality of valves in a short time, thus resulting in some valves undesirably left unclosed.
In order to overcome the above disadvantage, there has also been devised an arrangement which is adapted to automatically close the valve by the change-over with an electro-magnetic clutch through utilization of a restoring force of a spiral spring preliminarily wound manually. The above known arrangement, however, not only requires the troublesome procedure for manually winding the spiral spring in advance, but also involves a possibility of carelessly leaving the spiral spring unwound, and furthermore, owing to the necessity for employing the expensive electro-magnetic clutch, there is such a drawback that the manufacturing unit cost becomes extremely high, and thus, the arrangement has not been put into actual applications as yet.